This invention relates to a process for preparing an aqueous paint composition and to a coating process employing the composition.
U.S. Pat. No. 4,100,315 discloses a method for preparing aqueous coating powder dispersions containing as the binder a solid epoxy resin which is micronized in water in the presence of a specific combination of surfactants. For many applications, however, the presence of surfactants in the cured coating is not desirable.
European Patent Application No. 84201186.8 discloses the preparation of a solvent-free aqueous binder composition by a process which includes the steps of (a) dispersing pigment powder having a particle size less than 20 micrometers in a liquid, solvent-free epoxy resin, (b) dispersing the resultant paste in water, (c) reacting the resultant dispersion with a secondary amine, and optionally also a primary amine, the amount of amine or amines being sufficient to provide at least one N--H function per epoxy group of the epoxy resin, at a temperature in the range 40.degree. C. to 100.degree. C.; (d) neutralizing the resulting suspension of epoxy-amine adduct coated particles, and, optionally, (e) adjusting the concentration of the resulting dispersion within the range of 35 to 75 %w solids by addition of water.
By "solvent-free" is meant "free from organic solvents."
The above European patent application also discloses a process for preparing an aqueous paint composition which includes the steps of adding to the aqueous binder composition prepared by the described process a cross-linking agent which for practical purposes is mixed with pigment powder to form a paste, and adjusting the concentration of the composition by addition of water to ensure a solids content in the range 10 to 65 %w.
However, the above described process suffers from the difficulty of obtaining a homogeneous dispersion in step (b), and the high viscosity of the suspension obtained in step (c). Furthermore, the aqueous paint composition has a relatively low solids content at application viscosity.
The object of the present invention is therefore to provide a simplified method for preparing a solvent-free, aqueous epoxy resin-based paint composition.